We propose to study the irreversible gp120 binding as a new mechanism of HIV neutralization by antibodies. Preliminary studies suggest that the monoclonal antibodies (MAbs) raised to a covalently reactive analog of gp120 (gp120-CRA) express nucleophilic reactivity leading to binding activity with covalent character; neutralization of primary HIV isolates; and, a unusual epitope specificity. The proposal is composed of two specific aims: (1) characterization of epitopes and irreversible MAb binding to gp120 on the native viral surface and (2) study of the breadth and potency of neutralizing activity. Intact virions from diverse primary HIV isolates will be studied. Immunochemical endpoints and assays of viral infection in cultured peripheral blood mononuclear cells will be applied to objectively differentiate conventional reversible binding from irreversible virion binding and to assess the functional consequences of MAb-virus interactions. The studies are likely to strengthen the foundation for further development of neutralizing antibodies capable of permanent virus inactivation. [unreadable] [unreadable]